booting ipxe from pxelinux

[jwm77]

using pxelinux.0 (Version 4.04) I could boot ipxe in this way:

LABEL IPXE_shell
KERNEL /ipxe/IPXE.KRN ipxe command (eg. Shell)

using pxelinux (Version 6.01) this works no more
getting: "invalid argument" error.

same error occurs if I try

LABEL IPXE_shell
KERNEL /ipxe/IPXE.KRN
INITRD myscript.ipxe

Is there a workaround?
How to transfer a commandline to ipxe?
I perfer solutions without creating different configurations of ipxe because administration of versions is cumbersome.
pxelinux (v 5.x) I have never used